This disclosure relates to a manufacturing method for preparing polycarbonates, and more particularly, to a method for quenching the activity of residual catalyst employed in the manufacture of the polycarbonates.
Aromatic polycarbonates are used in a variety of applications due to their excellent mechanical and physical properties including, among others, impact and heat resistance, strength and transparency. There are three general processes known for the commercial manufacture of aromatic polycarbonates, which are illustrated in FIG. 1. The conventional interfacial process, as shown in FIG. 1A, and the phosgene-based melt process, as shown in FIG. 1B, start with the reaction of phosgene with carbon monoxide. The third general process, a xe2x80x9cno phosgenexe2x80x9d melt process as shown in FIG. 1C, was developed to eliminate the use of highly toxic phosgene in the process flow. Of these general methods, the xe2x80x9cno phosgenexe2x80x9d melt process shown is preferred since it prepares polycarbonates less expensively than the interfacial process and avoids the use of highly toxic phosgene.
Both types of melt processes (FIGS. 1B, and 1C) make use of a diarylcarbonate, such as diphenylcarbonate (DPC) as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A (BPA) in the presence of an alkaline catalyst to form a polycarbonate in accordance with the general reaction scheme shown in FIG. 2. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers. In many cases, however, the presence of residual catalyst in the finished polycarbonate has a detrimental affect on the quality of the product, leading to poor color, a decrease in molecular weight, a decrease in transparency, or undesirable rheological properties. Residual catalyst may also interact with additives, detracting from their efficacy and rendering the polycarbonate prone to thermal decomposition. Thus, it is desirable to reduce the levels of residual catalyst in the finished polycarbonate to minimize these interactions. Such reduction is referred to as xe2x80x9cquenching.xe2x80x9d
In the production of optical quality polycarbonates, several reactors are used in sequence to prepare the final product. The final reactors in this sequence subject the reaction mixture to both high temperature and high vacuum. This treatment assists in the removal of byproduct phenol, unreacted monomer and short oligomers, improving the overall quality of the final product. For optical quality products, this is also the phase of the reaction at which quencher is added. Because a very small amount of actual quencher is required, the quencher is usually added in a solvent.
A slight excess of the quencher is typically added directly to the polycarbonate. The quencher compound generally comprises an acid ester or an acid compound that is effective for deactivating or neutralizing the alkaline catalyst. However, it has been found that the acidic functionality of the quencher in the extruded or otherwise processed finished polycarbonate deleteriously affects hydrolytic stability. Discoloration and deterioration of other properties has been observed.
A process for quenching an alkaline catalyst used in a melt polycondensation reaction for the production of polycarbonate is described. The process includes combining an intermediate polycarbonate composition with a quenching composition in a reactor, wherein the quenching composition includes a compound having at least one acid or acid ester moiety and at least one amine moiety. The intermediate polycarbonate is heated and the pressure is reduced in the reactor to produce a finished polycarbonate. Advantageously, the use of an excess amount of the quencher compound to neutralize the alkaline catalyst does not deleteriously affect the properties of the finished polycarbonate.
These and other features will become better understood from the detailed description that is described in conjunction with the accompanying drawings.